1. Field of the Invention
This invention relates generally to a circuit for controlling the current flow through a coil, and more particularly is directed to an improved current control circuit for the horizontal deflection coil of a television receiver.
2. Description of the Prior Art
In a television receiver, the electron beam in a cathode ray tube is electromagnetically deflected by means of a deflection yoke positioned about the neck of the cathode ray tube envelope. Deflection circuits are provided to cyclically vary currents flowing in windings of the deflection yoke so that the resulting electromagnetic fields deflect the electron beam or beams to form a raster on the face of the cathode ray tube. In most instances, it is desirable that such raster be of rectangular shape. However, various distortions occur in the scanning of the face of the cathode ray tube by the electron beam and cause deviation of the configuration of the raster from the desired rectangular shape. One form of raster distortion is the so-called side pin-cushion distortion which results primarily from the physical geometry of the deflection system. Such pin-cushion distortion is well-recognized to be undesirable, and many arrangements have been proposed to correct or compensate therefor.
An arrangement proposed to correct or compensate for the pin-cushion distortion involves modulation of the power voltage supplied to the horizontal deflection circuit of the television receiver by means of a parabolic signal having a repetition rate equal to the vertical or field frequency of a video signal so that the horizontal deflection current is modulated by such parabolic signal. Usually, a variable impedance element in the form of a transistor is connected in series with the power supply path for effecting the desired modulation of the power voltage supplied to the horizontal deflection circuit and, in such case, a relatively large power consumption or loss is experienced in the transistor. Further, if the high voltage that has to be supplied to the anode of the cathode ray tube is obtained from the secondary winding of a fly-back transformer and the power voltage is supplied to the horizontal deflection circuit through a primary winding of such fly-back transformer, the modulation of the power supply voltage by the parabolic signal varying at the vertical rate results in the high or anode voltage obtained from the secondary winding also varying parabolically at the vertical rate so that a constant voltage cannot be obtained. The resulting variation in the anode voltage causes a corresponding change or variation in the brightness of the picture displayed on the cathode ray tube screen. Therefore, in order to obtain a constant high or anode voltage, it has been necessary to provide a high voltage generating circuit independent of the horizontal deflection circuit in the case where the above described arrangement is employed to correct for side pin-cushion distortion.
In another arrangement provided for dynamically correcting for side pin-cushion distortion, a saturable reactor is connected in series with the horizontal deflection coil or winding, and the reactance or inductance of the saturable reactor is modulated by the parabolic signal at the vertical rate so that the horizontal deflection current is modulated by such parabolic signal. However, due mostly to the characteristic of the saturable reactor, the linearity of the horizontal deflection current flowing through the horizontal deflection coil is deteriorated. Further, if the high or anode voltage is obtained from the secondary winding of the fly-back transformer which has its primary winding connected in series with the power supply path to the horizontal deflection circuit, variation of the inductance determining the retrace resonant frequency and, therefore, variation of the pulse width of the fly-back pulse, causes the high voltage obtained from the secondary winding of the fly-back transformer to vary parabolically at the vertical rate, as in the first described arrangement according to the prior art.